Photoresist compositions are used in microlithography processes for making miniaturized electronic components such as in the fabrication of computer chips and integrated circuits. Generally, in these processes, a thin coating of film of a photoresist composition is first applied to a substrate material, such as silicon wafers used for making integrated circuits. The coated substrate is then baked to evaporate any solvent in the photoresist composition and to fix the coating onto the substrate. The photoresist coated on the substrate is next subjected to an image-wise exposure to radiation.
The radiation exposure causes a chemical transformation in the exposed areas of the coated surface. Visible light, ultraviolet (UV) light, electron beam and X-ray radiant energy are radiation types commonly used today in microlithographic processes. After this image-wise exposure, the coated substrate is treated with a developer solution to dissolve and remove either the radiation exposed or the unexposed areas of the photoresist.
The trend towards the miniaturization of semiconductor devices has led to the use of new photoresists that are sensitive to lower and lower wavelengths of radiation and has also led to the use of sophisticated multilevel systems to overcome difficulties associated with such miniaturization. There are two types of photoresist compositions, negative-working and positive-working. When negative-working photoresist compositions are exposed image-wise to radiation, the areas of the resist composition exposed to the radiation become less soluble to a developer solution (e.g. a cross-linking reaction occurs) while the unexposed areas of the photoresist coating remain relatively soluble to such a solution. Thus, treatment of an exposed negative-working resist with a developer causes removal of the non-exposed areas of the photoresist coating and the creation of a negative image in the coating, thereby uncovering a desired portion of the underlying substrate surface on which the photoresist composition was deposited.
On the other hand, when positive-working photoresist compositions are exposed image-wise to radiation, those areas of the photoresist composition exposed to the radiation become more soluble to the developer solution (e.g. a rearrangement reaction occurs) while those areas not exposed remain relatively insoluble to the developer solution. Thus, treatment of an exposed positive-working photoresist with the developer causes removal of the exposed areas of the coating and the creation of a positive image in the photoresist coating. Again, a desired portion of the underlying surface is uncovered.
Photoresist resolution is defined as the smallest feature which the resist composition can transfer from the photomask to the substrate with a high degree of image edge acuity after exposure and development. In many manufacturing applications today, resist resolution on the order of less than one micron are necessary. In addition, it is almost always desirable that the developed photoresist wall profiles be near vertical relative to the substrate. Such demarcations between developed and undeveloped areas of the resist coating translate into accurate pattern transfer of the mask image onto the substrate. This becomes even more critical as the push toward miniaturization reduces the critical dimensions on the devices.
Photoresists sensitive to short wavelengths, between about 100 nm and about 300 nm can also be used where subhalfmicron geometries are required. Particularly preferred are photoresists comprising non-aromatic polymers, a photoacid generator, optionally a solubility inhibitor, and solvent.
High resolution, chemically amplified, deep ultraviolet (100-300 nm) positive and negative tone photoresists are available for patterning images with less than quarter micron geometries. Chemically amplified resists, in which a single photo generated proton catalytically cleaves several acid labile groups, are used in photolithography applicable to sub quarter-micron design rules. As a result of the catalytic reaction, the sensitivity of the resulting resist is quite high compared to the conventional novolak-diazonaphthoquinone resists. To date, there are three major deep ultraviolet (uv) exposure technologies that have provided significant advancement in miniaturization, and these are lasers that emit radiation at 248 nm, 193 nm and 157 nm. Examples of such photoresists are given in the following patents and incorporated herein by reference, U.S. Pat. No. 4,491,628, U.S. Pat. No. 5,350,660, and U.S. Pat. No. 5,843,624. Photoresists for 248 nm have typically been based on substituted polyhydroxystyrene and its copolymers. On the other hand, photoresists for 193 nm exposure require non-aromatic polymers, since aromatics are opaque at this wavelength. Generally, alicyclic hydrocarbons are incorporated into the polymer to replace the etch resistance lost by the absence of aromatics.
Photoresists based on chemical amplification mechanism are employed for 248 and 193 nm applications. However, the photoresist materials applicable for 248 nm cannot be used at 193 nm due to the high absorption of the poly (4-hydroxystyrene) based polymers used for 248 nm applications. 193 nm applications typically require non-aromatic compounds. Open-chain aliphatic resins cannot be used due to the very high etch rates of these materials. Polymers possessing annelated structures in the side chains such as tricyclododecyl and adamantane or cycloolefins in the main chain are shown to provide etch resistance close to poly(4-hydroxystyrene) polymers [Nakano et al. Proc. SPIE 3333, 43 (1998), Nozaki et al., Wallow et al. Proc. SPIE 3333, 92 (1998), and J. C. Jung et al. Proc. SPIE 3333, 11, (1998)].
Houlihan et al (U.S. Pat. No. 5,843,624), disclose a polymer for photoresist application, which is a polymer made from a cyclic olefin, maleic anhydride and a substituted or unsubstituted acrylate.
Comonomers containing lactone groups are disclosed in U.S. Pat. No. 6,447,980 and U.S. 2001/0044070. The present applicants have found that during processing conditions of the photoresists, the lactone group has a propensity for ring opening, which leads to crosslinking of the polymer. Thus there is a need to replace the lactone group with a more suitable group. The present applicants have also found that the replacement of the lactone group with the sulfone group can provide stability and improved hydrophilicity to the polymer, thus improving the lithographic performance of the photoresist. U.S. 2003/02196678 discloses fluorinated sulfone and sulfonated polymers for use as a 157 nm photoresist.
In another application of sulfone containing polymers it has been found that coatings of such polymers also possess high refractive index, In order to further improve the resolution and depth of focus of photoresists, immersion lithography is a technique that has recently been used to extend the resolution limits of deep uv lithography imaging. In the traditional process of dry lithography imaging, air or some other low refractive index gas, lies between the lens and the wafer plane. This abrupt change in refractive index causes rays at the edge of the lens to undergo total internal reflection and not propagate to the wafer. In immersion lithography a fluid is present between the objective lens and the wafer to enable higher orders of light to participate in image formation at the wafer plane. In this manner the effective numerical aperture of the optical lens (NA) can be increased to greater than 1, where NAwet=ni sin θ, where NAwet is the numerical aperture with immersion lithography, ni is refractive index of liquid of immersion and sin θ is the angular aperture of the lens. Increasing the refractive index of the medium between the lens and the photoresist allows for greater resolution power and depth of focus. This in turn gives rise to greater process latitudes in the manufacturing of IC devices. The process of immersion lithography is described in ‘Immersion liquids for lithography in deep ultraviolet’ Switkes et al. Vol. 5040, pages 690-699, Proceedings of SPIE, and incorporated herein by reference. It is also desirable for photoresists useful in immersion lithography to have high refractive index. The present inventors have found that photoresist polymers containing pendant sulfone groups from the polymer backbone provide a polymer with high refractive index. The refractive index of the polymer is proportional to the sulfone content in the polymer.
The present invention relates to a chemically amplified system, which is sensitive to wavelengths between 300 nm and 100 nm, and comprises a) a novel polymer that is insoluble in an aqueous alkaline solution and comprises at least one acid labile group, and, b) a compound capable of producing an acid upon radiation. The novel polymer of the invention when formulated into a photoresist, provides a photoresist with superior lithographic properties. The present invention also relates to a photoresist comprising an alkali soluble polymer, a dissolution inhibitor and a compound capable of producing an acid upon radiation.